1. Field of the Invention
The present invention relates to an optical glass for a polarizing optical system which is suitably usable for a polarizing optical system such as polarizing beam splitter and spatial light modulator for effecting polarizing modulation, and has an extremely small photoelastic constant, a process for producing such an optical glass for polarizing optical system, and a polarizing beam splitter utilizing the optical glass for polarizing optical system.
2. Related Background Art
In recent years, the utilization of a “polarizing characteristic”, as one of the factors constituting optical information, has rapidly been developed in various fields such as the field of liquid crystal. Along with such development in the utilization of the polarizing characteristic, in an optical system utilizing polarized light, i.e., a polarizing optical system, the importance of high-precision control of the polarizing characteristic constituting optical information has been increased year by year. Based on the increase in the above-mentioned importance, it has earnestly been desired to further improve the precision or accuracy in the control of the polarizing characteristic.
Among various optical elements constituting a polarizing optical system (such as substrate and prism), it is usual to use a material having an optical isotropy especially for some optical elements which are required to retain the polarizing characteristic. The reason for this is that when an optical element comprising a material having an optical anisotropy is used, the phase difference (optical path difference) between the ordinary ray and the extraordinary ray perpendicular to the ordinary ray will be changed during their passage through such a material, with respect to light which has been transmitted by the optical element, and therefore the polarizing characteristic cannot be retained in such a case.
In general, a glass which has sufficiently been subjected to annealing has an optical isotropy and also has various characteristics better than those of other materials in view of its durability, strength, transmittance, refractive index, cost, etc., and therefore such a glass is widely used for optical elements which should retain the polarizing characteristic. Particularly, borosilicate glass (e.g., a borosilicate glass mfd. by Schott Co., Germany, trade name: “BK7”) is inexpensive and excellent in durability, and also has little dispersion. Therefore, the borosilicate glass is widely used in many polarizing optical systems.
However, even when the above-mentioned conventional optical glass for polarizing optical system is used for the optical elements, a certain optical anisotropy based on a photoelastic effect can be induced in the optical element, under the application of a mechanical external stress or a thermal stress to the optical element. Accordingly, when the conventional optical glass is used for the optical element for a polarizing optical system, the polarizing characteristic of optical information can be changed on the basis of the “induced optical anisotropy” as described above. Therefore, in such a case, it is difficult for the polarizing optical system to exhibit a desired performance.
It is considered that the mechanical external stress and the thermal stress as described above are developed mainly in the following situation.
Thus, it is considered that the “mechanical external stress” is mainly developed in a step of processing a glass (such as cutting, the bonding or joining of the glass with another material, and film formation on the surface of a glass), or often a step of assembling a glass into an optical system (such as holding of the glass by a jig or holding device, and the adhesion of the glass to another member). In addition, it is considered that the “thermal stress” is developed by the production of heat in the interior of a glass (such as heat production based on the absorption of light energy), or the production of heat outside a glass (e.g., that based on heat production in a peripheral device) Further, when a glass is caused to contact or is joined with another material having a thermal expansion coefficient different from that of the glass, it is considered that a stress is developed along with the above-mentioned production of heat.
As described above, when a polarizing optical system is constituted by using an optical element, it has been difficult to completely obviate the action of the mechanical external stress or the thermal stress. Accordingly, when the conventional optical glass for polarizing optical system is used for such an optical system, it is extremely difficult to avoid the induction of the optical anisotropy based on the above-mentioned mechanical external stress or thermal stress.
An object of the present invention is to provide an optical glass for polarizing optical system, which does not substantially impair the polarizing characteristic of optical information, even under the action of a mechanical external stress or a thermal stress.
Another object of the present invention is to provide an optical glass for polarizing optical system, which is capable of controlling its refractive index in a desirable manner.